What are the types of optical design aberrations?

February 20, 2005

Aberrations are defects in images produced by a telescope (or optical system). They are caused by limitations in the design and manufacture of the optics. In theory, a perfect scope is possible, but in practice, it’s impossible to make a perfect telescope with real materials in real factories.

There are six kinds of aberrations (note that these aberrations can also appear in accessories like eyepieces):

Chromatic aberration is the failure of a lens or lens system to bring all colors to a common focus. It’s usually seen as a violet halo around bright stars and fringes around bright objects such as Jupiter or the Moon. It’s usually more pronounced in short focal ratio scopes. Prism diagonals and eyepieces can also significantly contribute to this type of aberration with your setup. Because it’s caused by the unequal refraction of light across the spectrum by just one kind of glass, it is greatly reduced in 2-element (two different glasses) achromats over simple lenses and even more so with apochromats (3 elements, 3 glasses). Use of extra-low dispersion (ED) glasses like fluorite is a common technique in lens design and manufacture to diminish chromatic aberrations. It doesn’t happen with mirrors. Catadioptric SCTs and Maks have very little chromatic aberration. Lateral color is chromatic aberration off the optical axis or on the edges of the field of view. It’s seen as colored blue and or yellow fringes away from the center of the field of view. A scope corrected for chromatic aberrations on axis can still have significant lateral color.

Spherical Aberration is the failure of rays passing at different distances from the center of a lens or mirror to come to the same focus. Edge rays will typically come to a focus closer to the lens or mirror than central rays. It’s sometimes called undercorrection, overcorrection or zonal errors. Again, low f-ratios are more affected, causing stars to look like blurred disks. Mirrors are often made with parabolic curves (Newtonians) or used with corrector plates (SCTs) to fix this aberration. For lenses, using the right shape for the lens reduces the error. For telescopes this is a lens or lens assembly with a more highly curved front surface (plano-convex) with the convex side facing the sky.

Coma is spherical aberrations from rays that come in off-axis. It shows up as little off-axis comet-shaped blobs that point inwards towards the center of the field and get bigger as you look towards the edge of the field of view. Fast parabolic mirrors show significant coma. It can be reduced at the same time spherical aberrations are reduced in the design of the main scope optics (types like the EdgeHD, corrected Dall-Kirkhams or Ritchey-Chretiens) or through the use of separate lenses in the optical train.

Astigmatism is another off-axis effect that causes rays from a star to come to different focal points in the vertical and horizontal plane. It elongates images horizontally and/or vertically, making them look cross-like. Its effects are especially pronounced when racking a star’s images through focus. Astigmatism is more likely to be found in very fast optical systems. It’s usually corrected with additional lens elements that vary spacing and stop down the optics.

Curvature of field occurs because an extended object or stars scattered in a field of view of a telescope won’t generally focus on a flat surface. A star at the center will be in focus, but stars farther out won’t be in focus. Almost all optical systems have some field curvature. Like astigmatism, it’s usually corrected with additional lens elements, varying spacing and stopping down the optics. Some photographic scopes (classic Schmidts) don’t try to correct it and instead use curved film holders to assure sharp focus.

Distortion is variable magnification across the field of view. It can be either pincushion (more magnification at the edge than at the center) or barrel (more at the center than at the edges) distortion. Correction of affected systems is achieved by stopping down the optics.